The present invention relates to an image forming apparatus, comprising a photosensitive belt carried or conveyed by a driving device, a plural number of developing means for forming toner images on said photosensitive belt, and an internal transcription means, on which the toner images on said photosensitive belt are transcribed, thereby forming a color image.
With an image forming apparatus, comprising: a photosensitive belt carried by a driving device.; a plural number of developing means for forming toner images on said photosensitive belt; an internal transcription belt, on the surface of which are transcribed the toner images on said photosensitive belt at plural times when forming the images for one page; and a transcription roller for pushing a paper onto said internal transcription belt, so as to transcribe the toner images to the paper, an apparatus is proposed, in which the photosensitive belt and the internal transcription belt are driven, as described in Japanese Patent Laying-Open No. Hei 8-328443 (1996) <JP-A 08-328443>.
Conventionally, in the image forming apparatus, wherein image forming for one page is conducted by putting the toner images formed on the photosensitive belt onto surface of the internal transcription belt at plural times, a driving mechanism is necessary, not only for carry of the photosensitive belt, but also for carry of the internal transcription belt, respectively. In such case, due to decentering (i.e., shift of the center) of gears, rollers, etc., of the respective mechanisms, difference occurs in velocity between the photosensitive belt and the internal transcription belt, thereby causing shift in piling up of colors as a result thereof, and therefore a driving system is necessary, having high accuracy.
If carrying the internal transcription belt by means of the photosensitive belt, i.e., to be driven thereafter, no such shift in piling up of colors occurs due to the difference of velocity of the driving system mentioned above. However, for performing such the carry of the internal transcription belt following after the photosensitive belt, with stability, it is necessary to widen nip widths thereof sufficiently. For that purpose, it is necessary to bring a photosensitive belt side roller, which pushes the internal transcription belt toward the photosensitive belt, to be large, up to a certain degree, in particular, in the diameter thereof. Also, if widening the nip width on the roller of small diameter, winding angle of the belt on the roller comes to be large, thereby increasing the surface tension due to bend of the photosensitive belt and the internal transcription belt, and there occurs creep deformation when being left for a long time. This deformation brings about gap shit between the photosensitive belt and the internal transcription belt when transcribing the toner, and also causing unevenness in concentration of the images. Even from such the viewpoint, it is necessary to keep the diameter of the photosensitive belt side roller to be large, in a certain degree.
At a marker position for measuring a cycle of the internal transcription belt, the velocity of the internal transcription belt is fluctuated upon ill influences given from the decentering of the photosensitive belt side roller. If periphery length of the photosensitive belt side roller is 1/N (N: an integer) of the internal transcription belt, this fluctuation in the velocity will not come to the periodic error. However, if making the photosensitive belt side roller large in the diameter thereof, for obtaining such the effect as was mentioned above, the ratio between the periphery length of the photosensitive belt side roller and the periphery length of the internal transcription belt does not comes to be such the integer, and thereby causing the periodic error. For this reason, if conducting an adjustment on motor speed for fitting the cycle to the standard time, it is done at the time of the carry velocity when the adjustment is inherently unnecessary, thereby causing the shift in piling up of colors.
When bringing the diameter of the photosensitive belt side roller and the periphery length of the internal transcription belt to be the ratio of integer, the periphery length of the internal transcription belt comes to be long, and therefore, a printing time for one page is elongated, as well as, the apparatus comes to be large the sizes thereof.